Considered the primary antagonist in the series, his name is derived from the French word for "thirteen". A calm, calculating man, Treize is a complex individual, playing both the chivalrous, anachronistic knight and the ruthless tyrant. Treize is a master politician and manipulator second to none. He is an expert strategist and tactician on and off the battlefield. In addition, Treize is a crack shot, a superb, Western-style fencer, and a mobile suit designer of great ability, having built both the Epyon and Tallgeese II to his own specifications. He is also an excellent mobile suit pilot, displaying his ability several times during the series, most notably during the final battle against Chang Wufei.

On the surface Treize is cold and reserved, rarely letting his emotions get the best of him. He professes a love for war, believing that when properly conducted, it brings out the best in humanity. He expresses an interest in honor and chivalry comparable to that of an old-fashioned aristocrat, believing that one should settle matters of honor in single combat, and considers himself a gentleman. This is juxtaposed with his ruthlessness in pursuit of his ambitions and his utter lack of scruples when it comes to holding onto power.

Beneath the surface, he is fanatically loyal to his friends. He dislikes hurting people, and suffers a great deal of guilt over the deaths he has caused. He also seems to have some difficulty recognizing, and dealing with, his own emotions. When Lady Une, who Treize has viewed as a pawn up until this point, is shot by Tsubarov, Treize is stunned by how strongly he reacts to it, realizing with a start that he may well have been in love with her all along. He experiences further difficulties when it comes to reconciling his personal feelings about war with his militaristic philosophy, which complicates his already complex life.

Eventually, Treize decides to step down as the leader of OZ as he is unable to support Romefeller's increased use of Mobile Dolls. This allows Duke Dermail takes direct control of the world. Later on in the series, Quinze and the White Fang begin a colonial rebellion against Romefeller and when Dermail ultimately ascends to space to lead the space troops, his shuttle is shot down, facilitating Treize's return to power. Treize comes out of exile, frees "Queen" Relena of her duties as sovereign of the world and assumes the position of Romefeller's chief representative. He then expertly rallies Romefeller and the rest of the world to gather arms and fight to defend the Earth from the White Fang, who, along with their new leader Milliardo Peacecraft, have declared war against the Earth.

Treize gathers his forces at the resource satellite MO-II and they fly toward the giant battleship Libra. Piloting the much refined and upgraded OZ-00MS2 Tallgeese II, Treize instructs his forces to stop, charges at Libra alone and challenges Milliardo to a duel. The White Fang leader refuses and fires Libra's cannon in an attempt to destroy Treize. The armies go to war and Treize downs numerous Mobile Dolls before he and Wufei find each other in the midst of the battle and engage in their second duel.

During their duel Wufei demands to know if Treize has any idea how many people have died for him. Treize lists the exact number, expressing regret at what he sees as sad, but necessary deaths. Treize then launches himself kamikaze-style at Wufei, leading to the Tallgeese II's cockpit being pierced by the Altron's beam trident. He tells Wufei it was an honor to fight him and the Gundam pilots and says,"I'll be waiting for you on the other side Milliardo," before his mobile suit explodes. At the end of Endless Waltz, Lady Une adopts his only daughter, Mariemaia.

Episode Zero

Treize appeared during Heero, Relena, and Wufei's portions of the story in the manga. In Heero's story which occurred in A.C.188, Trieze was shown to have originally been a piloting instructor for Mobile Suit pilots, who during the battle against the rebels in the colony he was at, protected his comrades including Lucrezia Noin who was a cadet at the time from a missile fired by a younger Heero Yuy. While in the hospital he met a woman by the name of Leia Barton who he became friendly with when she agreed with his comment that the alliance foolishly sent rain all over the colony to suppress a small fire. Leia would become the mother of his illegitimate daughter Mariemaia. During Relena's story in A.C.191, Treize accompanied Zechs in suppressing a small rebel faction in the J.A.P. area near the Darlian estate as ordered by OZ's leader at the time General Catalonia. While there he witnessed Zechs' skills for the first time and acknowledged the rumors about him moving fast as lightning were true.